Catalyst components for the stereospecific polymerization of olefins are widely known in the art. Concerning the polymerization of propylene, the most spread out catalyst family belongs to the Ziegler-Natta category and in general terms it comprises a solid catalyst component, constituted by a magnesium dihalide on which are supported a titanium compound and an internal electron donor compound, used in combination with an Al-alkyl compound. Conventionally however, when a higher cristallinity of the polymer is required, also an external donor (for example an alkoxysilane) is needed in order to obtain higher isotacticity. One of the preferred classes of internal donors is constituted by the esters of phthalic acid, diisobutylphthalate being the most used. The phthalates are used as internal donors in combination with alkylalkoxysilanes as external donor. This catalyst system gives good performances in terms of activity, isotacticity and xylene insolubility.
One of the problems associated with the use of this catalyst system is that the phthalates have recently raised concerns due to the medical issues associated with their use and some compounds within this class have been classified as source of heavy health problems.
Consequently, research activities have been devoted to discover alternative classes of internal donors for use in the preparation of catalyst components for propylene polymerization.
Some of the tested catalysts contain donors structures having contemporaneously amido groups and ester groups. WO2006110234 describes amino acid derivatives including one carbamate group and one free ester function. The catalysts generated by these structures have very low activity and sterospecificity in bulk propylene polymerization (table 2). In CN1104589 are described amino ester derivatives of formula R1-C(O)—O-A-NR2R3 in which R1 is a hydrocarbon group with 6-20 carbon atoms, particularly aryl or alkaryl; R2-R3 are hydrogen, C1-C6 alkyl, or R1; and A is a C1-C12 bivalent compound optionally substituted. Also in this case however, the activities/stereospecificity balance is very far from being acceptable for industrial exploitation.
Surprisingly, the applicant has found that a particular class of donors based on dicarbamates groups generates catalysts showing an excellent balance of activity and stereospecificity.